1. Technical Field of the Invention
This invention applies to automatic control systems for regulating and protecting radio frequency (RF) power amplifiers in transmitters and transceivers. In particular, it applies to controllers that receive feedback from several operating parameters of a power amplifier, including its direct current (DC) input, to perform closed-loop regulation.
2. Description of the Prior Art
Many radio transmitters include automatic control systems to regulate RF output power, to prevent amplifier damage from causes such as antenna load mismatch, excessive supply voltage, and excessive operating temperature, and to minimize additional damage after failure of one or more power devices.
FIG. 1 shows a typical RF power amplifier, 101, and power controller, 141, for a mobile transceiver. The power amplifier takes signals from an exciter, 103, amplifies them, and delivers them to an antenna, 111, or other RF load. The amplifier comprises a low-level driver stage, 105, and a high-power final stage, 107. The power amplifier receives its DC power through a cable, 119, coupled to a remote battery, 117, with ground return 123. The cable may have substantial distributed impedance. The power controller, comprising amplifier 143, frequency compensation capacitor 172, and buffer 145, receives control inputs, 161, and feedback signals, 151, and produces an output, 147, that varies the gain of the low-level driver stage.
The controller regulates output power during normal conditions and protects the amplifier during abnormal conditions by using negative feedback to diminish the error between the greatest feedback signal and a reference input that has been selected according to the nominal operating levels of the feedback transducers. In this example, several signals feed back from the power amplifier, including forward and reflected power indications, 113 and 115, respectively, from RF detectors coupled to the sampling arms of a directional coupler, 109; device temperature, 133, from a thermistor, 131, that is thermally coupled to the final amplifier devices; a differential voltage, 127-128, proportional to the amplifier's DC input current, 121, through a current-sampling resistor, 125; a signal, 129, that indicates the drive to the final amplifier; and a signal, 149, that feeds back the control voltage, 147, supplied to the driver. These feedback signals represent a typical design choice; other practical controllers contemplated within the scope of this invention would use these or similar signals.
Under normal conditions, all feedback signals except forward power are small. The controller increases drive until the forward power feedback signal becomes approximately equal to the reference input. Under abnormal conditions, another feedback signal will increase and exceed the forward power signal. For example, the reflected power reading increases when the antenna load becomes mismatched or removed. Increased RF drive without a corresponding increase in forward power indicates load mismatch or malfunction of one or more final devices. Excessive control voltage for a given output power indicates a problem in the low-level driver stage. Low DC input current indicates load mismatch or faulty driver or final devices. High DC input current or final device temperature indicates that the controller should reduce forward power demands on the amplifier. When one or more of these conditions occur, the controller reduces drive to the power amplifier to keep the largest feedback signal approximately equal to the reference input.
A difficulty in the prior art has been to design a generic controller to work with any of the amplifiers in a product line of a manufacturer, which may span different power levels and frequency ranges. Constraints in design of the directional couplers generally require that the reference voltage applied to the control amplifier be chosen according to the voltage at rated power from the forward power sensor; other feedback signals are scaled and translated to this common mode voltage. In particular, the feedback signal for DC input current must be translated from the battery voltage to the reference voltage applied to the non-inverting input of the control amplifier.
One prior art translation method is to sample the voltage across a series dropping resistor in the positive supply to the RF power amplifier with a voltage controlled current source driving a resistor coupled between the inverting input of the control amplifier and ground. This resistor typically develops only 100-300 mV at full current to avoid wasting voltage available to the amplifier. This design generally has poor common mode rejection, and the impedance of a long power cable, which may be as long as 22 feet in a mobile transmitter application, can affect stability of the control loop.